Recent advances in boiler technology have yielded high efficiency condensing boilers. A high efficiency condensing boiler has a primary heat exchanger and a secondary heat exchanger. Circulating fluid (such as return water) going through a high efficiency condensing boiler typically passes through the secondary heat exchanger and then enters the primary heat exchanger. The primary heat exchanger receives heat from burning fuel, while the secondary heat exchanger receives exhaust fumes from the primary heat exchanger for heat exchange with the circulating fluid. A temperature sensor sensed the circulating fluid entering the secondary heat exchanger and may be referred to as an “inlet” or “return” fluid temperature sensor. Circulating fluid entering the primary heat exchanger is monitored by another temperature sensor that is referred to as a “bypass” temperature sensor.
In many high efficiency condensing boilers, condensation in the primary heat exchanger can cause damage to the internal components of the primary heat exchanger. In such boilers, condensation should be minimized or prevented in the primary heat exchanger. Condensation is typically allowed to occur in the secondary heat exchanger, as the secondary heat exchanger is often constructed to withstand such condensation. In many cases, and to help minimize or prevent condensation from occurring in the primary heat exchanger, it is often desirable to keep the bypass water temperature above some minimum temperature.
The secondary heat exchanger typically causes, at most, about a ten degree rise in temperature of the returning fluid passing there through. To help keep the bypass temperature above the desired level, some high efficiency condensing boilers provide a bypass system that often includes a bypass valve that can be opened or closed, as needed, to maintain the bypass temperature at the boiler input by routing fluid leaving the primary heat exchanger back to the inlet to the primary heat exchanger.
Some applications include using a high efficiency condensing boiler to heat water that is supplied to circulation pipes under a sidewalk or driveway in cold climates (for example, to remove or prevent ice buildup). In such applications, hot water is supplied to the circulation pipes by the boiler, but a significant amount of heat is removed from the water before the water is returned to the boiler. As such, the water temperature at the inlet of the primary heat exchanger can be quite low, making it more difficult to maintain the bypass water temperature above the minimum temperature value in a robust and efficient manner.